According to the general theory of relativity, The force is not an covariant quantity under general coordinate transformations. Different coordinate conditions will lead to different damping forces. It will be convenent to find out the physical coordinate system in which the calculated value of the damping effect is the same as or nearest to that observed on earth. We expect that this physical coordinate system should be free from spurious non-transverse gravitational waves at infinity. In this coordinates the change of orbital period P due to emission of gravitational waves is given by P=-6.5×10-12(m1m2)/M⊙2((m1+m2)/m⊙)-1/3.